Generally, a rotating anode X-ray tube is used as a source of X-ray irradiation source of X-ray CT apparatus. This rotating anode X-ray tube mainly includes a cathode part and an anode part. The cathode part includes a filament which emits a heat electron, and a focusing electrode having focusing slot, positioned around the filament, which focuses the heat electron emitted from the filament on a target of the anode. The anode part is positioned opposite to the cathode part and including the target which is an umbrella-shaped, a rotation mechanism part which supports and rotates the target, and the fixed part which rotatably supports the rotation mechanism part.
In the rotating anode X-ray tube, a high voltage is impressed between the cathode part and the anode part to irradiate the X-ray from a focus of the target. Since a lot of heat is generated from the anode part in this case, an X-ray tube container (hereinafter referred as a tube container) is used as an inclusion body. In the tube container, the rotating anode X-ray tube is supported in an insulated oil to be insulated. The tube container includes an X-ray radiation window near the target of the rotating anode X-ray tube, and a cable receptacle to introduce the high voltage near the cathode part and the anode part.
A cathode side high-voltage cable is connected to the cable receptacle by-the side of the cathode, and negative high voltage and filament heating voltage for heating the filament are introduced. Moreover, an anode side high-voltage cable is connected to the cable receptacle by the side of the anode, and positive high voltage is introduced into it. However, an anode grounding type cable receptacle may be used instead. Moreover, a stator for rotating the anode part is attached near the rotation mechanism part of the anode part.
As mentioned above, the high voltage, such as 100 and dozens of kV, is impressed between the cathode part and the anode part, and the rotating anode X-ray tube generates the X-ray, when the heat electron emitted from the filament of the cathode part collides with the target of the anode part. The filament has a coil where a thin wire of an electronic radiation material, such as tungsten is winded, and a filament heating current is sent to the filament to be heated to a high temperature. From the heated filament, the heat electron of quantity corresponding to the temperature is emitted, and electric field formed by the high voltage impressed between the cathode part and the anode part accelerates the heat electron towards an anode part as an electron beam. At this time, the electron beam is focused on the target of the anode part as a desired size focus by the electric field formed by the focusing slot of the focusing electrode.
A flow of the electron beam is an X-ray tube current, and the high voltage impressed between the cathode part and the anode part is an X-ray tube voltage. Dose of the X-ray generated from the target becomes large when values of the X-ray tube current and the X-ray tube voltage are large. Moreover, the dose of the X-ray depends on the material of the target, and becomes large when the atomic number of the target material is large. However, since a generating efficiency of the X-ray in a range of the X-ray tube voltage used for X-ray imaging is so low, such as 1% or less, much energy caused by the electron beam which collides with the focus of the target is transformed into thermal energy. For this reason, the target of the anode part rotates at high speed by the rotation mechanism part, in order to avoid local overheating of the focus by the electronic beam.
As mentioned above, the target has a umbrella shaped face, and the X-ray generated from the target is emitted in a direction corresponding to an angle of inclination of the face (hereinafter referred as an target angle) which is an angle between the target face and a face perpendicular to an rotation axis of the target. The X-ray which passes through the X-ray radiation window is irradiated to a patient. The shape of the target is described in Japanese Patent Disclosure (Kokai) No2001-76657, paragraph 0016 to 0019, and FIGS. 1 and 2, for example.
The rotating anode X-ray tube is used for a multi-slice type X-ray CT apparatus which reconstructs a plurality of tomographic images along a body axis of the patient. The multi-slice type X-ray CT apparatus includes a multi detector having a plurality of detection element segments in a body axis direction (hereafter referred as a slice direction) of the patient, in order to reconstruct the tomographic images and to collect simultaneously the amounts of X-rays corresponding to the images.
However, in such the multi detector, difference of position of each detection element segment in the slice direction to the rotating anode X-ray tube becomes large. By the positional difference, difference of the apparent focal size, the dose and characteristic of the X-ray (hereafter referred as sensitivity change of X-ray characteristics) occurs, as shown in FIG. 9. Especially, in a new multi-slice type X-ray CT apparatus which has huge numbers of detection element segments, such as 128-or 256, since the difference of the position in the slice direction of each detection element segment become very large the sensitivity change of characteristics of the X-ray shall be considered. In an old multi-slice type X-ray CT apparatus where the number of detection element segments is 4 to 16, since the difference of the position in the slice direction of each detection element segment is not large, the sensitivity change of characteristics of the X-ray is not considered.
In the new multi-slice type X-ray CT apparatus, it is required that more exact projection data is collected in order to reconstruct the tomographic image in high resolution. In order to realize reconstruction in the high resolution, the X-ray which has an exact and uniform dose distribution is irradiated to the patient, and the projection data is collected by each detection element segment.
As described above, the dose distribution of the X-ray irradiated from the rotating anode X-ray tube is almost uniform in the body axis direction of the patient. However, the dose distribution in the slice direction of each detection element segment is a distribution which corresponds to the positional difference, and that is, the dose distribution has a gradual inclination. Therefore, quality of the tomographic image reconstructed based on the projection data detected by each detection element segment is nonuniform because of the difference of the dose distribution of the X-ray resulting from the difference of the position of the detection element segment.